Such devices and processes are needed, e.g., to detect the presence of certain care personnel at medical workplaces, such as intensive care units or operating rooms in order to control the medical devices depending on the presence of certain persons, e.g., to permit a change in certain settings of devices only in the presence of certain authorized persons. Other examples of application are found in the area of user identification in computer systems, e.g., to authorize data entry, which shall be performed by certain, identified persons only. Other examples of application are in the determination of the time during which personnel is present at certain workplaces, e.g., at intensive care units or hospital beds.
The authorization to operate a device or to control the behavior of a device proper as a function of the presence of a person usually requires an action by that person directed to this authorization. This authorization has been associated in the past with the entry of passwords or the insertion of machine-readable identification cards. The use of transponders in electromagnetic fields is an approach to the contactless transmission of information. The problems with these techniques are the relatively high cost of a detector, the usually large size of the antennas, the fault liability depending on the environment of the installation, and the like.
A second approach, which has found wide acceptance among consumers, is the use of infrared transmitters, which send a coded signal as a "key" to a device in order to activate or otherwise set this device. Such infrared transmitters are widespread in automobiles to trigger locking. The problem with this technique is that the user must hold the transmitter in his hand and must trigger it while directing it toward the receiver, which would be undesirable in many fields of application. Furthermore, the safety of such infrared keys against misuse is not sufficient.
An identification system, in which persons who wear a badge are tracked and identified, has been known from WO 93/18476. Each badge is provided with an infrared transmitter and a microprocessor, which ensures that an infrared signal unambiguously identifying the badge is sent periodically, e.g., every 10 sec. A plurality of receiver stations are distributed over the area to be observed, and the receiver station which has received the identifying infrared signal sends it to a central station. The location of the person in question can thus be tracked (based on the location of the receiver station) over time. The drawback of this process is that each identification unit or badge must continually send identification signals, which makes the identification unit to be worn by the user unwieldy because of the energy storage means it needs. Furthermore, sufficient safety against misidentification is not guaranteed, especially if a plurality of users are located in the vicinity of a receiver station.